The present invention relates to a semiconductor device. More particularly, the present invention relates to a method for forming a fine pattern in a semiconductor device.
The degree of integration of semiconductor devices depends on the fineness of patterns one can construct in the semiconductor devices. To increase the capacity of a memory chip, the size of the memory chip needs to be increased. However, the size of a cell region of the memory chip, on which fine patterns are formed, is practically decreased. Since more patterns are to be formed in the limited cell region to secure desired memory capacity, it is necessary to construct a fine pattern, such that the width of the fine pattern is less than the critical dimension. As a result, it is desired to develop a photolithography process for forming such a fine pattern.
In order to form a pattern by a photolithography process, a photoresist (“PR”) film is coated over a target layer to be patterned. Next, an exposure process is performed to change the solubility of the PR film at a given portion. Subsequently, a developing process is performed to form a PR pattern that exposes the target layer. The PR pattern is formed by removing the portion whose solubility has been changed, or by removing the portion whose solubility has not changed. Later, the exposed target layer is etched using the PR pattern, and then the PR pattern is stripped off to form a target layer pattern.
In the photolithography process, resolution and depth of focus (“DOF”) are two important measures. Resolution (R) can be expressed by Equation 1 below.
                              R          =                                    k              1                        ⁢                          λ              NA                                      ,                            (        1        )            where k1 is a constant determined by the material and the thicknesses of the PR film, λ is the wavelength of a light source, and NA stands for the “numerical aperture” of an exposure equipment.
Since k1 has a physical limitation, it is difficult to reduce k1 by an existing method. Thus, there is a need to develop a new exposure equipment that employs a light source having a narrow band, and a new photoresist material that effectively responds to the new exposure equipment. Without such a development, it is difficulty to form a fine pattern in semiconductor devices.
FIGS. 1a to 1c are cross-sectional views illustrating a conventional method for forming a fine pattern in a semiconductor device by using a double exposure process. As shown, a target layer 20, a hard mask layer 30, and a first photoresist film (not shown) are sequentially formed on a semiconductor substrate 10. The first photoresist film is exposed and developed using a line/space mask (not shown) to form a first photoresist pattern 40. Hard mask layer 30 is etched using first photoresist pattern 40 to form a first hard mask pattern 30a that exposes first portions of target layer 20. First photoresist pattern 40 is then removed.
Referring to FIGS. 1b and 1c, a second photoresist film (not shown) is formed on first hard mask pattern 30a and the exposed portion of target layer 20. The second photoresist film is exposed and developed using a line/space mask to form a second photoresist pattern 45. Second photoresist pattern 45 exposes portions of first hard mask pattern 30a, the exposed portions of first hard mask 30a being located substantially around the centers of two adjacent first portions of target layer 20. First hard mask pattern 30a is patterned using second photoresist pattern 45 to form a second hard mask pattern 32, thereby exposing second portions of target layer 20. Second photoresist pattern 45 is then removed. Target layer 20 is etched using second hard mask pattern 32 to form a target pattern 20a. 
According to the above described method, it may be difficult to form a fine pattern due to the resolution limit of the exposure equipment. In addition, there may be a misalignment issue between two patterns that are formed by the two-step exposing process to overcome the resolution limit.